Pathogens, defences and vaccinationIB MYP Biology: Revision notes
Section 1
Pathogens and how they spread
A pathogen is a microorganism that causes disease. There are four main types:
- Bacteria, e.g. cholera, tuberculosis and Salmonella food poisoning
- Viruses, e.g. influenza, measles and HIV
- Fungi, e.g. athlete's foot and ringworm
- Protists, e.g. malaria, carried by mosquitoes
Pathogens are transmitted by droplets in the air (coughs and sneezes), by contaminated food and water, by direct contact, by body fluids and by vectors such as mosquitoes.
Section 2
The body's first defences
The body has barriers that stop pathogens from getting in:
- The skin is a physical barrier, and scabs seal cuts.
- Mucus in the nose and airways traps pathogens, and cilia move it away from the lungs.
- Stomach acid (hydrochloric acid) kills most pathogens that are swallowed.
Section 3
White blood cells
If pathogens get inside the body, white blood cells attack them in three ways:
- Phagocytosis: phagocytes engulf pathogens and digest them.
- Antibodies: lymphocytes make antibodies that match the antigens on the surface of one type of pathogen. The antibodies destroy the pathogen or make it clump together.
- Antitoxins: they neutralise toxins that pathogens release.
Antibodies are specific. An antibody for the measles virus will not work against influenza.
Section 4
Vaccination and herd immunity
A vaccine contains a weakened or dead pathogen, or its antigens. It makes white blood cells produce antibodies and memory cells, so the body responds faster and more strongly if the real pathogen infects later.
When a high proportion of a population is vaccinated, the pathogen cannot spread easily. This is herd immunity, and it protects people who are not vaccinated, such as babies.
Benefits: fewer cases and deaths, and some diseases can be eliminated. Risks: mild side effects such as a sore arm or fever, and rarely an allergic reaction. No vaccine is 100% effective.
Section 5
Antibiotics and resistance
Antibiotics kill bacteria but do not work against viruses, because viruses live inside body cells.
Bacteria can become resistant to antibiotics:
- A mutation makes some bacteria resistant.
- The antibiotic kills the non-resistant bacteria.
- The resistant bacteria survive and reproduce.
- The resistant strain spreads and the antibiotic no longer works.
Overusing antibiotics, or not finishing a course, makes resistance more likely.
Must Know
- Four types of pathogen: bacteria, viruses, fungi, protists
- Defences: skin, mucus, stomach acid
- White blood cells: phagocytosis, antibodies, antitoxins
- Vaccines cause antibody and memory cell production; herd immunity protects others
- Antibiotics do not work on viruses; overuse causes resistance
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Pathogens, defences and vaccination
- A public health team in Dhaka is investigating four illnesses in a district: cholera, influenza, athlete's foot and malaria. Each one is caused by a different type of pathogen and each one spreads in a different way.Describe how cholera is transmitted and outline one way of preventing its spread.2 marks
- A student in Lima falls on rough ground and cuts her knee. That evening she also breathes in dust and swallows some food that contains a few bacteria. Over the following days she stays healthy and the cut heals.Outline how the skin and the mucus in the airways help to prevent infection.2 marks
- Amira is 14 and lives in Amman. She has a fever and a headache. Her doctor tells her that the illness is caused by a virus and decides not to prescribe antibiotics, although her mother asks for them. Amira's white blood cells have already started to respond to the infection.Describe three ways in which white blood cells defend the body against pathogens.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).